Making a crater on a sphere surface
I'd do away with sculpting, create it from a base icosphere instead.
Add an icosphere, subdivide it as necessary (about 2 in the example below), then on top of it add a Subdiv Modifier and set shading to smooth.
Then in Edit Mode select clusters of contiguous faces, spaced wherever you want craters to be.
Press E to extrude them with the option Along Normals, followed by a small Inset for the ridges, then another final Extrude Along Normals to create the depression.
Repeat for more craters, you can adjust crater radius either by insetting once before the first extrusion, or by scaling the extruded faces after the first extrusion.